Steel & Timber

Ferrous metals

Ferrous metals contain iron as the main constituent; their properties depend largely on carbon content.

Types of cast iron

• Grey cast iron — graphite flakes; machinable, brittle; most common castings.
• White cast iron — carbon as cementite; very hard and brittle.
• Malleable cast iron — white iron heat-treated — some ductility.
• Ductile (spheroidal graphite, SG) iron — graphite nodules (by magnesium addition) — much tougher and ductile — ductile iron pipes.

Classification of steels by carbon

Effect of increasing carbon: strength and hardness increase; ductility, toughness and weldability decrease.

Manufacture of steel

Iron ore is reduced in a blast furnace with coke and limestone to produce pig iron (hot metal). Steel is then made by removing excess carbon and impurities:

• Bessemer process (historic) — air blown through molten iron.
• Open hearth process (historic).
• Basic oxygen furnace (LD process) — pure oxygen blown onto molten iron — major modern route.
• Electric arc furnace (EAF) — melting scrap (and sponge/direct reduced iron) with electric arcs — flexible, used widely in India.
• Induction furnace — small-scale melting of scrap and sponge iron.

Molten steel is refined (secondary metallurgy), continuously cast into billets, blooms and slabs, and hot rolled into bars, sections, plates and coils.

Properties of mild steel

Stress–strain curve (tension)

For mild steel:

1. Proportional limit — stress proportional to strain (Hooke's law).
2. Elastic limit — full recovery on unloading.
3. Upper and lower yield points — sudden extension at nearly constant stress (yield plateau).
4. Strain hardening — stress rises again with further strain.
5. Ultimate tensile strength — maximum stress.
6. Necking — local reduction of area.
7. Fracture — breaking point (ductile cup-and-cone fracture).

High-strength deformed (HSD/TMT) bars and cold-worked steel usually have no definite yield point; the 0.2% proof stress (stress at which permanent strain of 0.2% remains) is taken as the yield strength.

Reinforcing steel

Types

Grades (IS 1786)

High-strength deformed bars are produced in grades Fe 415, Fe 415D, Fe 415S, Fe 500, Fe 500D, Fe 500S, Fe 550, Fe 550D, Fe 600 (the number is the minimum 0.2% proof stress/yield stress in N/mm²).

• "D" grades have higher ductility requirements (greater elongation and higher ratio of tensile strength to yield strength) — preferred for earthquake-resistant structures; "S" grades are for special seismic applications.
• Indicative minimum elongation: about 14.5% for Fe 415, 12% for Fe 500, and higher for D grades (e.g. about 16% for Fe 500D); minimum ratio of tensile strength to yield strength about 1.10 for Fe 415 and 1.08 for Fe 500, with higher ratios for D grades.
• Bend and rebend tests check ductility of bars.
• Deformations (ribs/lugs) improve bond with concrete.

Structural steel

• IS 2062 — hot-rolled medium and high tensile structural steel — grades such as E250 (yield 250 MPa), E350, E410, E450 (with quality sub-grades A, BR, B0, C related to impact properties and weldability).
• Rolled sections: ISMB (medium weight beams), ISMC (channels), ISA (angles), ISJB, ISLB, ISHB, T-sections, plates, flats, tubes (hollow sections).
• Prestressing steel — high-tensile plain wires and stranded wires (e.g. 7-wire strands) with tensile strengths commonly in the range of about 1500–1900 MPa, low relaxation.
• Weathering steel (e.g. Corten type) — forms a stable protective rust patina — bridges, facades.
• Stainless steel — contains at least about 10.5% chromium (often with nickel) — forms a passive chromium oxide film — corrosion-resistant reinforcement, fittings, cladding.
• Cold-formed light gauge sections — purlins, light frames.

Heat treatment of steel

Corrosion of steel

Corrosion is the electrochemical deterioration of steel in the presence of moisture and oxygen, forming rust (hydrated iron oxides), which occupies several times the volume of steel — causing cracking and spalling of concrete cover.

Accelerated by: chlorides (sea water, deicing salts), carbonation of concrete (loss of alkalinity), acids, industrial pollution, stray currents, contact with dissimilar metals (galvanic corrosion), cracks and inadequate cover.

Prevention

• Protective coatings — paints (primers such as zinc chromate, red oxide), epoxy coatings (fusion-bonded epoxy-coated bars), bituminous coatings.
• Galvanising — zinc coating (sacrificial protection).
• Metallic coatings — zinc/aluminium metallising, electroplating (chromium, nickel).
• Cathodic protection — sacrificial anodes (zinc, magnesium, aluminium) or impressed current systems — pipelines, marine structures, bridge decks.
• Corrosion-resistant steels — stainless steel, weathering steel.
• In concrete: adequate cover, dense low-permeability concrete (low w/c), limiting chlorides, corrosion inhibitors, crack control, blended cements.
• Design details — avoiding water traps, ensuring drainage and ventilation, avoiding dissimilar metal contact.

Non-ferrous metals

Alloys

Classification of trees

Softwoods and hardwoods

Structure of a tree (cross-section)

Felling of trees

• Trees are felled when mature (maturity varies from a few decades to over a century depending on species); immature timber has more sapwood, and over-mature timber may have decay.
• Felling in a season when sap is at its minimum is preferred (to reduce decay and speed seasoning) — commonly mid-winter in plains and mid-summer in hilly regions.

Qualities of good timber

Sound (clear ringing sound when struck); uniform colour and straight, close grain; free from defects (knots, shakes, decay, insect attack); adequate strength, hardness, toughness and elasticity; durability against fungi and insects; well seasoned (appropriate moisture content, no warping); heavy for its species (denser timber is stronger); fibrous and not brittle; sweet smell (fresh cut); good workability and ability to hold nails and screws; fire resistance for large sections.

Seasoning of timber

Seasoning is the controlled drying of timber to reduce its moisture content to a level in equilibrium with the atmosphere where it will be used.

Objectives

• Reduces shrinkage, warping and cracking in service.
• Increases strength, hardness, stiffness and durability (more resistance to decay).
• Reduces weight (economical transport).
• Makes timber easier to work, paint, glue and treat with preservatives.
• Improves resistance to fungi and insects.

Freshly felled (green) timber may contain moisture exceeding its dry weight; well-seasoned timber commonly has about 10–15% moisture content (lower for furniture in dry interiors, somewhat higher for outdoor/structural use).

Methods

Conversion (sawing) of timber

Conversion is sawing logs into marketable sizes (planks, battens, scantlings).

Defects in timber

Defects due to natural forces

Defects due to fungi

• Dry rot — fungi attack timber in damp, poorly ventilated, warm conditions (e.g. floor boards on damp walls); timber becomes dry, brittle and powdery; can spread through masonry.
• Wet rot — decay under alternate wetting and drying (e.g. posts at ground level); timber becomes soft and spongy.
• Brown rot, white rot, heart rot, sap stain, blue stain.

Defects due to insects

Termites (white ants) — hollow out timber internally; beetles (powder-post beetles) — reduce sapwood to powder; marine borers (shipworms) — attack timber in sea water.

Defects due to seasoning

Defects due to conversion

Chip mark, diagonal grain (improper sawing), torn grain (tool action), wane (original rounded surface/bark on the edge of a sawn piece).

Preservation of timber

Objectives: increase durability and life; protect from fungi, insects and marine borers; reduce fire hazard (with fire retardants).

Preservatives

Methods of application

• Brushing and spraying — surface treatment; limited penetration; maintenance treatments.
• Dipping and steeping — immersion for minutes to days.
• Charring — surface burnt to form a protective charcoal layer (e.g. ends of posts in the ground).
• Hot and cold open tank process — timber heated in preservative, then cooled so that preservative is drawn in.
• Pressure impregnation — in closed cylinders:
  • Full-cell (Bethell) process — vacuum then pressure — maximum retention (marine piles).
  • Empty-cell processes (Rueping and Lowry) — deep penetration with less retention (economical).
• Boucherie process — sap displacement in green poles.

Fire resistance

Timber is combustible, but large sections perform relatively well in fire: the surface chars, and the char layer insulates the inner wood, which retains strength. Fire resistance is improved by fire-retardant chemicals (e.g. ammonium phosphate, ammonium sulphate, borax, boric acid), fire-retardant paints, larger sections, and protection with plaster or boards.

Properties of timber

• Anisotropic — properties differ parallel and perpendicular to the grain: compressive and tensile strengths parallel to grain are much higher than perpendicular to grain.
• Strength decreases with increasing moisture (below fibre saturation point) and with defects (knots, sloping grain).
• Density — denser timber is generally stronger and harder.
• Good strength-to-weight ratio, low thermal conductivity (good insulator), good sound absorption, easy to work.
• Shrinkage: tangential shrinkage > radial shrinkage > longitudinal shrinkage (negligible).

Common Indian timbers

Wood-based products

Measurement of timber

Worked examples

Frequently tested points

• Exogenous (softwoods, hardwoods) vs endogenous (bamboo, palm).
• Softwoods: conifers, distinct rings, resinous, light (deodar, pine); hardwoods: broad-leaved, strong (teak, sal, shisham).
• Heartwood — dead, durable; sapwood — living, less durable; cambium forms new wood; medullary rays radial.
• Seasoning reduces moisture (well-seasoned about 10–15%); fibre saturation point ~25–30%.
• Kiln seasoning — quick and controlled (compartment and progressive kilns).
• Quarter and radial sawing reduce warping; flat sawing is economical.
• Shakes: heart, cup (ring), star, radial; knots reduce strength.
• Dry rot — damp unventilated conditions; wet rot — alternate wetting and drying.
• Seasoning defects: bow, cup, twist, checks, splits, honeycombing, case hardening.
• Preservatives: creosote (oil type), CCA/ASCU (water-borne); pressure impregnation — Bethell (full-cell), Rueping and Lowry (empty-cell).
• Large timber sections char and retain strength in fire.
• Strength much higher parallel to grain; tangential shrinkage > radial > longitudinal.
• Plywood: odd number of veneers, cross-grained; glulam, LVL (parallel grains), CLT (cross layers); compreg = compressed impreg.
• Quarter girth formula $V=(G/4{)}^{2}L$.